The United States Federal Communications Commission (FCC) has approved a digital high definition television (HDTV) standard proposed by the Grand Alliance (GA), clearing the way for terrestrial digital television broadcasting in the United States. The GA HDTV system adopted the Motion Pictures Expert Group (MPEG2) video compression standard. For details, see “Information Technology—Generic Coding of Moving Picture and Associated Audio Information: Video,” ISO/IEC 13818-2: 1996(E). Utilizing modern and sophisticated video compression methods, such as source processing, motion estimation and compensation, transform representation and statistical coding, an MPEG compression system can reduce the transmission bit rate by a factor of 50 or more. A full HD signal for one second requires approximately one billion bits before compression. As proposed in the GA specification, 1920 by 1080 pixel (picture element) images at a rate of 60 field per second are compressed to 18 megabits per second for digital broadcast.
The GA video compression system normally comprises two major subsystems, a pre-processor and an MPEG2 video encoder, followed by an output buffer. The input to the pre-processor is analog video in RGB format. The pre-processor digitizes the input signals and performs gamma correction on each color component to compensate for the non-linear response of the imaging camera. Gamma correction reduces the visibility of quantization noise contained in the compressed image, especially in the dark regions of the image. Then the pre-processor linearly transforms the digitized and gamma corrected RGB samples to the SMPTE 240M YC1C2 color space. Finally, the resulting chrominance components are sub-sampled to form the 4:2:0 digital video input signal. In addition to the tasks just described, the pre-processor may perform image conversion. For example, in a broadcast digital satellite system, the video signal is horizontally decimated from 720 pixels per line to 544 pixels per line to further reduce bandwidth requirements. This signal is sent to the MPEG2 video encoder.
The MPEG2 video encoder compresses the input digital video signal by removing some of the temporal redundancy between frames and some of the spatial redundancy within frames. Generally, compression is achieved by utilizing a number of different techniques in succession as described above. Adjusting the quantization precision allows the encoder to generate the compressed bit stream at any rate specified by the application. The quantization in MPEG2 systems is performed on the DCT coefficients of a data block, which may be the original image information or residual information from motion estimation. Using quantization matrices in combination with scalable quantization step sizes, the quantizer selects and quantizes only a small fraction of the DCT coefficients from every DCT block for transmission, resulting in significant data reduction. The quantization matrices can be varied on a frame basis according to the statistical distribution of the DCT coefficients and the content of the video signal. For different areas within a frame, the quantization can be fine tuned on a macroblock by macroblock basis by scaling the quantization step size based on the complexity of the macroblock. For a given output bit rate, an output buffer will provide control signals used by the encoder to adjust the quantization step size for a particular frame to maximize quantization resolution within the available bandwidth.
Ideally, the video compression system removes high frequency components which will not be perceived as missing by viewers when the image is reconstructed and displayed. The remaining low frequency components are quantized to fit within the available bandwidth. Quantization noise introduced to the signal should also be invisible to viewers upon image reconstruction. However, in a real system a trade-off is made between the information to transmit and the quantization step size for an available bandwidth. If the system does not drop sufficient coefficients for quantization, the system increases the quantization step size resulting in blocky artifacts in the reconstructed image. If the picture loses too much high frequency information during the compression process, the reconstructed image will contain other noticeable edge artifacts.
Moreover, differences in quantization between each frame causes frames within a group of pictures (GOP) to contain varying high frequency components. An I frame, for example, may have a substantial amount of high frequency coefficients dropped during encoding, whereas P and B frames retain high frequency components corresponding to those dropped in the I frame. The reconstructed GOP will now contain artifacts because the high frequency information varies between frames used to reconstruct each other.
These problems occur within the GA system as currently defined. Compressing the HD image signal further only decreases displayed picture quality. Satellite broadcast providers are unwilling to transmit HD signals because only one program can be transmitted at a time in a transponder. To date, compressing an HD program sufficiently to fit two programs in one satellite channel (for example, 24 MHz 4-PSK Modulation) at the same time results in unacceptable viewer picture quality. Therefore, satellite broadcast providers are hesitant to broadcast HDTV due to inefficient use of the channel. Likewise, terrestrial broadcast providers are reluctant to commit to providing full HD programs when one program fully occupies a channel in which several SD programs can reside.